Stability Flashcards
In what state of equilibrium would a vessel return to the upright?
Stable equilibrium, due to having a positive Righting lever, by a positive GM
What is FSE?
The energy created by the free movement of liquids on deck or in tanks
How do we minimise FSE?
Longitudinal tanks on build
Baffles in tanks
Completely slack or full tanks
Ensure all water freeing arrangements are clear and functional
Ensure camber of decks is appropriate
Explain a Stiff vessel and its characteristics:
Large GZ/small KG
Larger righting lever
Difficult to incline initially
Returns to upright quickly
Uncomfortable for crew
Undue stress on vessel and cargo
Racking stress
Explain a Tender Vessel and its characteristics:
Small GZ/Large KG
Easy to incline initially
Smaller righting lever
More comfortable for crew and passengers
DANGERS OF SYNCHRONOUS ROLLING
Chance of Dangerous Gv through fuel consumption
What is an angle of List?
An angle achieved by the horizontal movement of G
What is an angle of Loll?
An angle achieved by the virtual movement of G
If the vessel is heeled, it will remain at the angle of heel achieved by the force
If the force is continuously applied, it will continue to move
Duties by OOW/CO in preparations for Dry Docking:
All hatches and Beams stowed to ensure continuity of strength throughout ships length
All derricks and cranes stowed
FSE removed from tanks/decks
Stability calculations carried out to ensure adequate GM, allowing for the rise in G when the vessel takes the blocks
Inform Dock authorities of the Dry docking plan
Sound all tanks to ensure similar stability when leaving dry dock
Lock up lavatories before entering the dock
Ensure fenders rigged and dock shores are placed against strength members once vessel is positioned
Take fire mains, electricity, gangway and nets ASAP
What is a docking plan?
A Docking Plan is a profile view of the vessel, that includes information on vessel particulars, positioning of keel blocks/ dock shores, bottom and bilge blocks etc. To be given to dry docking party and discussed prior to dry docking procedure.
Preparations for heavy weather: WT integrity
Hull openings closed
WT doors closed
Hawse pipes and Spurling pips covered
Gooseneck vent pipes covered
Unnecessary Sea Valves closed
Preparations for heavy weather: Vessel Security
Deadlights/Storm shutters
Scuppers/freeing ports/fairleads clear
Extra lashings/correct stowage
FSE countered for
Lowering of large weights if possible
Emergency comms tested
Preparations for heavy weather: Crew and passenger safety
Restrict Deck access
Rig Lifelines
Harnesses/life jackets worn
Modify watch schedule
Have galley prepare easy food
What is Gross Tonnage?
The internal volume of the vessel less certain exemptions
What is Net tonnage?
The earning capacity of the vessel (cargo holds etc)
What is Reserve Buoyancy?
Volume of enclosed watertight spaces between the Load Line and the freeboard deck.
The Reserve of Buoyancy must prevent the deck line submerging when the largest watertight compartment is bilged.
Formula for FWA
1/48 of Summer Draught
or
Displacement / 4 x TPC (SW)
How to Use a Hydrometer
Measures the Relative density of a fluid (In this case, Dock water)
Use a clean bucket of water
Fill with dock water
Allow water to stand
Ensure Hydrometer is clean and dry
Place in water and spin gently to remove air bubbles
Allow water to settle
Read Density at bottom of the Meniscus
Dry and pack away Hydrometer
Buoyancy
An upward force Exerted by a fluid that opposes the weight of a partially or fully immersed object
Centre of Buoyancy
Point where the total buoyancy force is considered to move upwards. This is the centroid of the ships underwater volume
Centre of Gravity
Point where the total weight force of the ship is considered to act vertically downwards
Transverse metacentre
Point of intersection between force line through B, and centre line at a small angle
How is the Density of a substance defined
The density of a substance is defined as its mass per unit volume
Define Relative Density
A ratio of the density of a substance compared to that of Fresh water.
FW 1.000
SW 1.025
Dock water lies in between
Displacement
Mass of the vessel at any given moment floating at any draught between light displacement & Summer draught marks.
Load displacement
Mass of the vessel fully loaded with cargo, etc. floating at her assigned freeboard.
Deadweight
Difference between lightship displacement and displacement at any given draught.
Tonnes per Centimetre Immersion
The mass required to load/discharge in order to change the mean draught by 1cm. It is
used to derive ‘change in draught’ over small increments.
TPC = (Waterplane Area/100) x Density
Righting Lever
Horizontal Distance (in metres) between the vertical line of buoyancy, acting through B, and the ships centre of gravity when the ship is heeled.
Z is the point on the vertical line of buoyancy force when it is perpendicular to the centre of gravity
Righting Moment
The result of the ships displacement (buoyancy force) acting against the end of the righting Lever (GZ)
Righting Moment (t-m) = Δ (t) x GZ (m)
Formula for Dock water allowance
DWA = FWA x (1025 - Dock water density) / 25
Minimum legal allowance for GM for a yacht on arrival in port
0.15m
What are the SIX points of a Load Line?
(Ensuring Adequate Reserve Buoyancy)
- Physical strength of a vessel when submerged
- Wetness of weather deck
- Permeability weather deck
- Safety of crew on deck
- Sufficient Stability when intact
- Stability in damaged condition, following PRESCRIBED damage and ASSUMED flooding
